A thermoelectric generator (TEG) utilises a temperature difference occurring between a hot (warm) object, i.e. a heat source, and its colder surrounding, i.e. a heat sink, and is used to transform a consequent heat flow into a useful electrical power. The necessary heat can be produced by radioactive materials, as e.g. in space applications, or by sources available in the ambient, like e.g. standard cooling/heating systems, pipe lines including pipe lines with warm waste water, surfaces of engines, parts of machinery and buildings or by endotherms (i.e. by warm-blooded animals including human beings and birds, as well as by other endotherms). Natural temperature gradients also could be used, such as geothermal temperature gradients and temperature gradients on ambient objects when naturally heating/cooling at day/night, etc.
There is a growing commercial interest in small-size TEGs, which could replace batteries in consumer electronic products operating at low power and in autonomous devices. For example, TEGs mounted in a wristwatch have been used to generate electricity from wasted human heat, thus providing a power source for the watch itself, see M. Kishi, H. Nemoto, T. Hamao, M. Yamamoto, S. Sudou, M. Mandai and S. Yamamoto in “Micro-Thermoelectric Modules and Their Application to Wristwatches as an Energy Source”, Proceedings ICT'99 18th Int. Conference on Thermoelectrics, p. 301-307, 1999. Also, the first wireless sensor nodes fully powered by TEGs have been practically demonstrated and successfully tested on people as reported by V. Leonov, P. Fiorini, S. Sedky, T. Torfs and C. Van Hoof in “Thermoelectric MEMS generators as a power supply for a body area network”, Proceedings of the 13th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers'05), Seoul, Korea, Jun. 5-9, 2005, pp. 291-294; by B. Gyselinckx, C. Van Hoof, J. Ryckaert, R. Yazicioglu, P. Fiorini and V. Leonov in “Human++: Autonomous Wireless Sensors for Body Area Networks”, Proc. of the Custom Integrated Circuit Conference (CICC'05), 2005, pp. 13-19; and by V. Leonov and R. Vullers in “Wireless Microsystems powered by homeotherms”, Proc. Smart Systems Integration Conference, Paris, 27-28 Mar. 2007. Also the first practically useful device for medical applications, a wireless pulse oximeter, has been demonstrated which is fully powered by a wrist TEG and does not contain any battery as reported by T. Torfs, V. Leonov, B. Gyselinckx and C. Van Hoof in “Body-Heat Powered Autonomous Pulse Oximeter”, Proc. of the IEEE Int. Conf. on Sensors, Daegu, Korea, 22-25 Oct. 2006, see also in Abstract book, p. 122.
Recently, MEMS technology has also been used to fabricate miniaturised thermopiles, as described by M. Strasser, R. Aigner, C. Lauterbach, T. F. Sturm, M. Franosh and G. Wachutka in “Micromachined CMOS Thermoelectric Generators as On-chip Power Supply”, Transducers '03. 12th International Conference on Solid State Sensors, Actuators and Microsystems, p. 45-48, 2003 (Infineon Technologies); by A. Jacquot, W. L. Liu, G. Chen, J.-P. Fleurial, A. Dausher, B. Lenoir in “Fabrication and modeling of an in-plane thermoelectric micro-generator”, Proceedings ICT'02. 21st International Conference on Thermoelectrics, p. 561-564, 2002; and by H. Bötner, J. Nurnus, A. Gavrikov, G. Kühner, M. Jägle, C. Künzel, D. Eberhard, G. Plescher A. Schubert and K.-H. Schlereth in “New Thermoelectric Components using Microsystem Technologies”, Journal of Microelectromechanical Systems, vol. 13, no. 3, p. 414-420, 2004.
Recently, thin film technology has also been used to fabricate miniaturised TEGs on a thin polymer tape, as described by S. Hasebe, J. Ogawa, M. Shiozaki, T. Toriyama, S. Sugiyama, H. Ueno and K. Itoigawa in “Polymer based smart flexible thermopile for power generation”, 17th IEEE Int. Conf. Micro Electro Mechanical Systems (MEMS), 2004, pp. 689-692; by I. Stark and M. Stordeur in “New micro thermoelectric devices based on bismuth telluride-type thin solid films”, Proceeding of the 18th International Conference on Thermoelectrics (ICT), Baltimore, 1999, p. 465-472; and by I. Stark in “Thermal Energy Harvesting with Thermo Life®”, Proceedings of International Workshop on Wearable and Implantable Body Sensor Networks (BSN'06), 2006.
Recently, thin-film technology has also been used to fabricate miniaturised thermopiles on a membrane, where the membrane is a thin layer of material suspended on and sustained by a carrier frame, the membrane being much thinner than the carrier frame. Miniaturised thermopiles on a membrane are e.g. described by A. Jacquot, W. L. Liu, G. Chen, J.-P. Fleurial, A. Dauscher, B. Lenoir in “Fabrication and modelling of an in-plane thermoelectric micro-generator”, Proceedings ICT'02. 21st International Conference on Thermoelectrics, p. 561-564, 2002.
In the patent application US-2006-0000502, a micromachined TEG is proposed specially suited for application on heat sources having large thermal resistance, e.g., on human beings. It is proposed and shown that an effective TEG for such applications should contain a large hot plate, a large radiator and a tall spacer somewhere in between the plates. The design and technology for the first micromachined thermopiles specially suited for such applications are reported by V. Leonov, P. Fiorini, S. Sedky, T. Torfs and C. Van Hoof in “Thermoelectric MEMS generators as a power supply for a body area network”, Proceedings of the 13th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers '05), 2005, pp. 291-294.
Recently, an effective TEG using any of the above-mentioned thermopile types has been proposed, with specific thermal matching arrangements implemented in the TEG and/or with a multi-stage arrangement of the thermopiles, offering further improvement of its performance on a heat source or/and on a heat sink with high thermal resistance, more specifically when the TEG is used under conditions of non-constant heat flow and non-constant temperature difference (U.S. Ser. No. 12/028,614).
TEGs can be characterised by an electrical and a thermal resistance and by both voltage and power generated per unit temperature difference between the hot and cold sides of the TEG. The relative importance of these factors depends on the specific application. In general, the electrical resistance should be low and, obviously, voltage or power output should be maximised (in particular in applications with small temperature difference between the heat source and the heat sink, i.e. a few degrees C. or few tens degrees C.). If a constant temperature difference is imposed at the boundaries of the TEG, e.g. by means of hot and cold plates at fixed temperatures relative to each other, the value of thermal resistance is not crucial, because the output voltage and the output power are proportional to the temperature difference, which is fixed. Contrary thereto, if the boundary condition is a constant heat flow or a limited heat flow through the device, then the thermal resistance is of primary importance and the voltage and the power produced by the TEG are different from the voltage and the power produced under conditions of constant temperature difference. The term “constant heat flow” means that in the considered range of TEG thermal resistances the heat flow through the device is constant (limited by the ambient). However, this does not mean that the heat flow stays at the same value over time in a practical application. The term “limited heat flow” means that when decreasing the thermal resistance of the TEG, the heat flow through the device increases till a certain value, at which the conditions of constant heat flow are reached. In the case of “limited heat flow” the heat flow through the device is not limited by the ambient, but is limited for example by the thermal resistance of the TEG.
The basic element of a TEG is a thermocouple 10 (FIG. 1). An example of a thermocouple 10 comprises a first thermocouple leg 11 and a second thermocouple leg 12 formed of two different thermoelectric materials, for example of the same but oppositely doped semiconductor material and exhibiting low thermal conductance and low electrical resistance. For example, the thermocouple legs 11, 12 could be formed from BiTe. If the first thermocouple leg 11 is formed of n-type BiTe, then the second thermocouple leg 12 may be formed of p-type BiTe, and vice versa. The thermocouple legs 11, 12 are connected by an electrically conductive interconnect, e.g. a metal layer interconnect 13, which forms a low-resistance ohmic contact to the thermocouple legs 11, 12. The points of contact in between the legs 11, 12 and interconnects 13 are called thermocouple junctions.
In FIG. 2, a TEG 20 comprising a thermopile 21 comprising a plurality of, preferably a large number of thermocouples 10, is shown. The thermopile 21 is sandwiched in between a hot plate 22 and a cold plate 23. The hot plate 22 and the cold plate 23 are made of materials having a large thermal conductivity, so that the thermal conductance of the plates 22, 23 is much larger (at least by a factor of 10) than the total thermal conductance of the thermopile 21.
In case of a heat source or/and a heat sink with high thermal resistance, three types of thermopiles and their arrangement in a TEG may be considered as suitable: (1) commercial small-size thermopiles arranged in a multi-stage structure according to U.S. Ser. No. 12/028,614, (2) a micromachined thermopile on a raised elongated structure or on a spacer according to US-2006-0000502, (3) a thermopile on a polymer tape arranged as e.g. reported by Ingo Stark and P. Zhou in WO 2004/105143, by Ingo Stark in US 2006/0151021 and by 1. Stark and M. Stordeur in “New micro thermoelectric devices based on bismuth telluride-type thin solid films”, Proceeding of the 18th International Conference on Thermoelectrics (ICT), 1999, p. 465-472. Membrane-type thermopiles with a thermal difference between the center of the membrane and its side frame (A. Jacquot, W. L. Liu, G. Chen, J.-P. Fleurial, A. Dauscher, B. Lenoir in ‘Fabrication and modeling of an in-plane thermoelectric micro-generator’, Proceedings ICT'02. 21st International Conference on Thermoelectrics, p. 561-564, 2002) are not appropriate for applications on a heat source and/or on a heat sink with high thermal resistances because of their thermal mismatch (due to their small contact area with the heat source or the heat sink), and consequently the too low voltage and power they would produce.